Fixed networks can be used for distributing audiovisual services to receivers with a coaxial cable and/or an optical cable, radio networks or distribution via satellite as a physical transmission link. As an example of a fixed network, FIG. 1 illustrates a schematic view of one cable TV system. In the system a central site 1 is referred to as Head End where the received transmission is distributed to several physical signal paths, in this case to various optical fibres on which the emission is carried closer to consumers. The optical signal near a consumer is converted into an electric signal in a converter 2 and conducted to a coaxial cable having several branch circuits. In the branch circuit the signal is amplified and the amplified signal is then conducted to several receivers, that is, the coaxial cable is branched to several homes and it is further connected to the receiver. Downstream, that is, from the network equipment towards the terminal information is transmitted encrypted or non-encrypted. The frequency band of the system may extend as far as 1 GHz while the channel width is typically 8 MHz. Upper limit frequency is determined by the characteristics of amplifiers 3 (FIG. 1) placed in the cables branching from the coaxial trunk cable. One analog channel or about ten digital channels can be placed in this channel width of 8 MHz if the transmission rate is 38 Mbits, in which case 10 MPEG2 encoded video transmissions can be sent on one band of 8 MHz. Downstream control channels are placed in the frequency range of 70 to 130 MHz. Upstream control channels are placed in the frequency range of 5 to 55 MHz.
A method can be used in this system where both downstream and upstream transmission take place in time slots which are numbered starting from zero and ending in some figure max., after which the numbering may start again. ATM cells, for example, can be transmitted in time slots. Time slots o, . . . , max. can be thought to form a frame. In order that the terminals would be able to send information upwards, the time slots where they are allowed to send have to be reported to them in some way. The simplest way is to report in a separate signalling message to each terminal the time slots where it is allowed to send. A disadvantage of this method is that allocation of time slots by terminal equipment signalling messages is rather slow as there may be hundreds or even thousands of terminals. If numbering has to be changed considerably, a new signalling message has to be sent to all terminal equipment, which is a slow process.
A broadcast system has been presented in the field where the downstream frame structure is based on ITU-T recommendation G.704 (1544 kbit/s). The present application introduces the use of a frame of recommendation G.704 comprising 256 bits (32 bytes) numbered from 1 to 256, the repetition rate of the frames being 8000 Hz, in which case the bit rate will be 2048 kbit/s. The first eight bits of the frame have specified values depending on whether or not the frame contains a frame synchronization signal. The values of bits in a byte are shown in the table of FIG. 2A. Bit S.sub.i is reserved for international use but within the country its use is free, wherefore it is used in frames not containing the synchronization signal in multiframe structures and in CRC-4 error correction. Bit A is reserved e.g. for a remote alarm indicator and bits S.sub.a4, S.sub.a5, S.sub.a6, S.sub.a7 and S.sub.a8 are reserved for future use.
In the way presented by the Applicant, downstream time slot DSS in Broadcast system will be formed of two consecutive frames according to G.704, FIG. 3A. The first byte of the first frame is indicated by SB0 and its bits are synchronization bits, i.e. S.sub.i, 0, 0, 1, 1, 0, 1, 1 intended for the frame included in the frame synchronization of Table 1A. The payload is placed in the remaining 31 bytes of the frame. The first byte of the second frame is indicated by SB1 and the bits are the bits of FIG. 2A which are intended for the frame not containing the frame synchronization signal. This is followed by 22 bytes intended for the payload and at the end, the CONTROL field intended for downstream control bits and the FEC field intended for error correction bits. The lengths of the last two fields can be selected freely. A DSS time slot thus comprises 64 bytes and the duration of a time slot is 250 microseconds. A downstream signal is a constant rate bit stream where frames follow one another and 4,000 cells are transmitted per second.
Eight consecutive time slots DSS form a multiframe as shown in FIG. 3C. The duration of a multiframe is 2 milliseconds. The time slots are numbered from 0 to 7. The structure of the multiframe according to G.704 recommendation is shown in the table of FIG. 2B. The first four DSS time slots, that is, frames 0 to 7, form the first part SMF 1 (Sub-Multiframe) of the multiframe and frames 8 to 15 the second part SMF II. The table shows the first bits of each frame. The first byte of even-numbered frames contains the frame synchronization bits shown in the upper part of FIG. 2a, each first bit C.sub.1, . . . C.sub.4 forming a bit group of CRC-4. In SMF I the bits of the first byte of odd-numbered frames are as shown in the lower part of FIG. 2A when the first bit is 0. In SMF II the first bit in frames 9 and 11 is 1, whereas the first bit E in frames 13 and 15 is permanently one. At the beginning the value of the indicator is 0.
Upstream frame structure and time slots, that is, the direction where the terminal equipment sends information, frame structure and time slots to the network are examined now. Upstream transmission rate may be 2048 kbit/s or 256 kbit/s. An upstream signal is formed of 63 byte frames and after the frame there is an empty guard byte. FIG. 4 shows upstream time slots, indicated by USS0, . . . . USS7. The length of an upstream time slot has to be such that it fits inside a downstream time slot, that is, it is the same as or shorter than the upstream time slot. For the sake of clarity, a downstream multiframe has been described at the top of the figure, upstream time slots in the middle when the bit rate is 2048 kbit/s, and upstream at the bottom when the bit rate is 256 kbit/s which low bit rate is caused by sending only bits of one time slots, that is, 63 bytes in the duration 2 ms of the multiframe. The terminals have to take propagation delay into consideration in the transmission so that the head-end (FIG. 1) will see the upstream and downstream time slots exactly as shown in FIG. 4.
In the system, there are 1 to 4 upstream OOB control channels for one out of band control channel OOB (Out of Band Channel) if the bit rate is in both directions the same 2048 kbit/s, or 1 to 32 upstream control channels if the upstream bit rate is 256 kbit/s. It is also possible that there are both 2048 kbit/s and 256 kbit/s control channels in the upstream. In that case the downstream control channel controls the timing and access of the upstream control channels associated with it.
In the upstream and downstream frame structures explained above information may be transmitted in ATM cells of UNI type (user-network interface). At the ITU-T, it has been agreed that an ATM cell consists of a header of 5 bytes and an information portion of 48 bytes. The header contains in the user-network interface seven fields, the first of which fields is a 4-bit flow control field GFC which is used in the flow control of varying services together with flow control functions of an in-house network. Other fields are e.g. virtual path identifier VPI and virtual channel identifier VCI.
An ATM cell in downstream is placed in the payload of two consecutive frames as shown in FIG. 3B for which reason two consecutive frames, in which the ATM cell is transmitted, are logically referred to as time slot DSS.
In upstream, where the frame is 63 bytes, an ATM cell can be transmitted in the frame as in FIG. 5A in which there is at the beginning a preamble of 4 bytes, after this the ATM cell as a whole sent by the terminal and last an error correction field FEC of 6 bytes. The duration of one byte is reserved as guard time.
It has been proposed in advanced systems similar to the one described above that the time slots in both directions are numbered consecutively 0, . . . 4095, after which numbering is started from the beginning. Two channels are presented for the upstream in one of which the access mode is Aloha, in which case all subscribers may send in any time slot. The network acknowledges the successful transmission by echoing on a downstream channel. On the second upstream channel the terminal is allocated the time slot intended only for it, which slot is valid continuously, for example, upstream time slots 2 and 200 are continuously available for the same subscriber. A disadvantage of this arrangement is that it is difficult to change the designated time slots flexibly and the terminal reserves a time slot even if there is nothing to be sent. The upstream access mechanism is therefore not very efficient.
The present invention relates to an effective upstream access mechanism of a control channel which is suitable for use in any data transmission systems where distinct frames or time slots may be found in the transmission of information.
The present upstream access method is characterized by. what is stated in the independent claims.